Method for forming fiber-reinforced composite

ABSTRACT

Fiber reinforced composite products of enhanced interlaminar ply strength and processes of producing same. In forming the products, layers of fibrous reinforcing material and resinous matrix material are established. The matrix material contains unset resin and short fibers of an average particle length of 0.2-0.6 mm in a weight ratio of short fibers to resin in the range of 0.4-1. The reinforcing material comprises fibers of a length substantially greater than the length of the short fibers in the matrix material. The fiber and matrix layers are integrated in order to cause the resin and short fibers which comprise the strengthened matrix material to enter into the interstitial spaces of the longer fiber reinforcing material. Thereafter, the matrix resin is solidified in order to arrive at the fiber reinforced composite product. Alternate fiber and matrix layers may be built up on a surface while a pressure gradient is established from the outside of the built up layers to the surface in order to cause the resin to flow across the interfacial boundaries of the resin and fiber layers. The flow across the resin fiber interfaces promotes orientation of the short fibers in a direction across the interfaces so that the short fibers enter into the interstitial spaces between the reinforcing fibers.

TECHNICAL FIELD

This invention relates to fiber reinforced composite products and moreparticularly to composite products of enhanced interlaminar strength andmethods for the production of such products.

ART BACKGROUND

High performance composite structures such as are used in formingsurface skin components of aircraft and space vehicles usually areformulated of fiber reinforced plastic compositions. Such materialsinclude glass fibers such as E-glass or S-glass fibers, boron or carbonfibers, and aramid fibers such as the products identified as Kevlarfibers. The plastic matrix materials useful in the formation of suchcomposites include polyester and epoxy resins, polyimides, polyamides,polybutadiene resins and vinyl ester polymers. Such materials can becured by cross linking at temperatures ranging from room temperature upto about 400° F. to 600° F. or by application of chemical cross linkingagents. Where the structures are to be subjected to high temperatureconditions as in the case of radomes for high performance aircraft andheat shields for space vehicles, thermosetting resins such as thosedescribed above usually will be employed in the matrix material. Inother cases, where less severe conditions are to be encountered,thermoplastic resins can be employed as the matrix material although inmost cases thermoset resins are used.

In forming products having relatively simple shapes such as flat sheetsor rectangular polyhedra, layup and bagging techniques can be preparedwith a fair degree of success. For example, Kirk-Othmer, "Encyclopediaof Chemical Technology," 3rd Edition, 1981, Supplement Volume at pages268-270, discloses a process for preparing a flat composite product byplacing a carbon-fiber-epoxy prepreg layup on a flat tool surface. Thelayup is covered with breather plies and a nylon bag is placed over thebreather plies and sealed at its edges to the tool surface. A vacuum ispulled upon the sealed assembly in order to evacuate air from the layup.The assembly is placed in an autoclave, heated and pressurized to effecta cure cycle.

A similar approach has been proposed for use in molding threedimensional composite structures. For example, as disclosed in U.S. Pat.No. 3,962,394 to Hall, a cylindrical or rectangular tubular mandrel iscoated with a resin fiber layer which is surrounded by a compressionsleeve formed of a thin film of nylon or rubber which is perforated withholes and split lengthwise. A layer of absorbent material is placedaround the split compression sleeve and this assembly is surrounded by aplastic bag or bladder which is sealed at both ends to the tubularmandrel. The bladder is evacuated in order to cause the compressionsleeve to compact the layers and expel trapped air and excess resin fromthe fiber-resin material through the holes in the compression sleeve.

Typically, in forming plastic composite products 5-10, sometimes more,plies of fiber mats are integrated together with interveningapplications of resinous material matrix material to arrive at the finalproduct. Each fiber ply comprise fibers which are oriented generally ina two dimensional surface of the ply, e.g., in a plane in the case of aflat composite product or in the wall of a cylinder in the case of acylindrical product. The fiber ply may be of a continuous strand type asin the case of a filament wound on a molding tool. Alternatively, thefiber material may be formed of a plurality of generally parallelcontinuous fibers, e.g. in the form of an "unidirectional tape", or itmay comprise chopped fibers aligned in an unidirectional manner. Suchchopped fibers typically are of a length within the range of 0.1-3centimeters.

In addition, the fiber mat may take the form of a braided structure inwhich the fibers extend predominantly along one direction but arebraided or woven together, normally to provide an angle between strands,the "braid angle" of about 15°-45°. A fabric type structure in whichfibers are interconnected by cross-strands intersecting at about 90° mayalso be employed. The fiber layers may be in the form of prepregs inwhich the fibers are impregnated with an uncured resin which is latercrosslinked in order to provide matrix material.

As described in Kirk-Othmer, Encyclopedia of Chemical Technology, 3rdEdition, 1981, Vol. 13, pp. 968-978 under the heading "Laminated andReinforced Plastics" and in the Supplement Volume, pp. 260-281, underthe heading "Composites, High Performance," various procedures areavailable for forming fiber reinforced plastic composites. A basicapproach involves a technique in which fiber layers, which may or maynot be prepregs containing resin, are disposed on a forming tool andresin, usually an uncured thermosetting resin, is either sprayed orpainted on the fiber layers. Additional fiber layers and resin layersare added until the desired thickness is achieved and the resultinglayup is then cured to produce the final product. The layup structurecan be squeezed together under a light force in order to force the resinand fibers into intimate contact. Curing can take place under an appliedpressure. Other processes useful for forming cylindrical productsinvolve the winding of a fiber filament around an internal mandrel. Thefilament is wound onto a rotating mandrel and resinous matrix materialapplied, either by running the filament through a tank of uncured liquidresin or by spraying or painting the liquid onto the fiber filament asit is disposed on the mandrel.

In some composite products, it is desirable that the composite fiberplies be in close juxtaposition to one another with the resin materialintimately mixed therewith. In other procedures, pronounced layering orlamination occurs. For example, U.S. Pat. No. 4,269,884 to DellaVecchiaet al discloses a process of forming a stampable thermoplastic sheetwhich comprises several more or less discrete layers. In theDellaVecchia procedure, outer layers are formed of a thermoplastic resinwhich may optionally contain up to 50% of a particular filler and up to45% of nonsiliceous fibers having a length ranging from about 0.01 to3/4 of an inch. The fibers are generally oriented two dimensionally in aplane parallel to the plane of the sheet. Inside of the outer layeranother resinous sheet is provided. This is in a molten state during theprocessing procedure to allow the internal fiber mats to be impregnatedby the resin. Fiber mats are disposed upon an internal supportingscreen. The fiber and resin layers are passed through rollers whichapply a pressure to the sheets of between 1000 to 1500 pounds per linearinch to ensure bonding of the several layers and impregnation of thefibers by the adjacent thermoplastic molten resin. The DellaVecchiaprocedure is carried out in a manner to prevent migration from one layerto the next, specifically the process is carried out to avoid migrationof the long reinforcing fibers to the outer resinous layer and also toavoid migration of the short fibers, if present in the outer resinlayer, into the reinforcing layer.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a new andadvantageous process for the formation of fiber reinforced compositematerials of enhanced interlaminar ply strength. In carrying out theinvention, layers of fibrous reinforcing material and unset resinousmatrix material are established. The matrix material contains unsetresin and short reinforcing fibers having an average particle lengthwithin the range of 0.2-0.6 mm. The fibers are present in the matrixmaterial to provide a weight ratio of short fibers to resin in the rangeof 0.4-1. The reinforcing material comprises fibers, which may be anysuitable form, for example, a continuous strand, unidirectional choppedfibers, or woven fibers. The reinforcing fibers are of a lengthsubstantially greater than the length of the short fibers in the matrixmaterial. Reinforcing fibers typically will range in size from about 3cm up to the length of the product, for example, in the case ofunidirectional tape layups. In the case of filament wound around arotating mandrel or other molding tool, the fiber strand, of course, maybe continuous. The fiber and matrix layers are integrated in order tocause the resin and short fibers which comprise the strengthened matrixmaterial to enter into the interstitial spaces of the longer fiberreinforcing material. Thereafter, the matrix resin is solidified inorder to arrive at the fiber reinforced composite product.

In a further aspect of the invention, a plurality of layers of resinousmatrix material and fibrous reinforcing material are provided. Thelayers of matrix material contain liquid resin and short reinforcingfibers. The layers of reinforcing material contain fibers having anaverage fiber length substantially greater than the average fiber lengthof the short fibers in the matrix material. The fiber layers and amatrix material layers are integrated by alternately disposing thematrix layers and the fiber layers on a forming surface. The formingsurface may be of any desired configuration. For example, it may be acylindrical or conical mandrel, a concave or convex shape such as informing aircraft panel components, or simply a flat planar surface. Asalternate fiber and matrix layers are built up on the surface, apressure gradient is established from the outside of the built up layersto the forming surface in order to cause the resin to flow across theinterfacial boundaries of the resin and fiber layers. The pressuregradient may be initiated at the conclusion of stacking of the fiber andresin layers or it may be initiated at an intermediate point in thebuildup process. In either case, the flow across the resin fiberinterfaces, promotes orientation of the short fibers in a directionacross the interfaces so that the short fibers enter into theinterstitial spaces between the reinforcing fibers. At the conclusion ofthe buildup procedure, the resin is solidified to produce the fiberreinforced composite material. Solidification can take place duringpressurization of the built up layers or the pressure may be releasedand the material then solidified.

Yet a further aspect of the invention involves a fiber reinforcedcomposite product. The product comprises a plurality of plies of fiberreinforcing mats formed of elongated fibers oriented generally along thetwo dimensional surface dimensions of the reinforcing mats. The mats maytake the form of braided structures or unidirectional oriented fiber,e.g. as formed by a continuous strand or by a plurality of generallyparallel strands. In addition, the fiber reinforcing mats may comprisechopped fibers, either aligned unidirectionally, or braided, or evenrandomly oriented. The composite further comprises a resin materialproviding a matrix in which the fiber reinforcing mats are disposed.Short interlocking fibers having an average length within the range of0.2-0.6 mm are disposed in the resin in a three-dimensional orientationand extend into the interstitial spaces between the elongated fibers ofthe reinforcing fiber plies.

A further application is in the formation of shaped composite structuresbased upon thermoset resins which are subject to shrinkage when they arecrosslinked and particularly in producing such structures formed ofseveral components which are molded separately and then bonded together.The mating components are formed from a plurality of layers of fiberreinforcing material of elongated fibers which are integrated with aresinous matrix material comprising an uncured thermoset resin and shortreinforcing fibers. The components are then cured to crosslink thethermoset resin to cause the components to solidify in the desiredshapes. The component parts are then joined together along theirconforming mating surfaces to provide the final shaped compositeproducts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing a plurality of alternatelydisposed layers of fiber mats and resin materials built up on a formingsurface.

FIG. 2 is a schematic sectional illustration showing short fiberorientation resulting from an imposed pressure gradient across thelaminar plies used in producing the composite buildup.

DETAILED DESCRIPTION OF THE INVENTION

As noted previously, various procedures may be used in forming fiberreinforced composite materials. Such general procedures as thosedisclosed in the aforementioned Kirk-Othmer articles may be employed incarrying out the present invention, provided they are implemented in amanner to arrive at a well-integrated structure in which the fiber pliesare in close juxtaposition to one another to provide a relativelyhomogeneous cross section in which the matrix material is infused withinthe fiber plies without resin rich laminates between the fiber plies.Such structures may be contrasted with highly laminated structures suchas those produced by procedures as disclosed in the aforementionedDellaVecchia patent, which are carried out in a manner to retardmigration between successive layers.

The various resin and fiber materials disclosed in the aforementionedKirk-Othmer articles also may be employed in carrying out the invention.The invention is particularly applicable to thermoset composites, i.e.,materials which are cured at the conclusion of the layup procedure bycross linking. However, the invention may also be carried out employingthermoplastic composites in which the resinous matrix materials areretained in the thermoplastic state. When employing thermoplasticresins, the compositing procedure is carried out at temperatures abovethe thermoplastic melt points of the resins involved, and solidificationof the resinous matrix material to arrive at the composite isaccomplished by cooling the structures to appropriate temperatures wellbelow the processing points. Fiber-resin formulations of both thethermoset and thermoplastic type are disclosed in the aforementionedKirk-Othmer articles "Composites, High Performance" pp. 260-281 of theSupplement Volume and "Laminated and Reinforced Plastics, pp. 968-978 ofVol. 13. The entire disclosures of these Kirk-Othmer articles areincorporated herein by reference.

In fiber reinforced composite structures, the fiber reinforcing pliesare the predominant load bearing members and the resinous matrixmaterials function primarily to hold the fiber plies together. In theidealized case, complete penetration of the matrix resin into the fabricplies occurs with little or no resin enriched zones between the plies.Even when the idealized case is approached, a significant fatigue pointin composite structures is interlaminar failure between the fiber plies.The present invention provides composite structures of substantiallyenhanced interply strength. This is accomplished through the use of amatrix resin containing extremely short fibers which function inconjunction with the resin to interlock the fiber layers together.

In experimental work carried out respecting the invention, compositetest panels were prepared using various concentrations of short fibersof different lengths. The test panels were formed on 6"×24" aluminumplates by braiding continuous fiber strands on the aluminum plates withan application of a resin and short fiber mixture between the fiberplies. Each fiber ply was composed of two successive braided layers. Thebraid angles, as measured with respect to a braid axes extendinglongitudinally of the aluminum mandrel were usually 30°, although in afew cases braid angles of 15° were employed. The resin base used for thematrix was an epoxy resin identified as Epon 815 from Shell ChemicalCompany. The hardener for the resin was HY-906 and the accelerator,DY-062, both from Ciba Geigy Corp. The fiber employed was a continuousstrand of E-Glass yarn available from Owens Corning FiberglassCorporation. The resin mixture comprised 55 wt % of the Epon 815, 44% ofthe DY-062 accelerator and 1% of the HY-906 hardener.

The test specimens were prepared using fiber-resin mixtures containingshort fibers ranging up in concentration up to 50 wt. % (a ratio offibers to resins of 1) and having nominal lengths of 1/4", 1/32" and1/64". (6.4 mm, 0.8 mm, and 0.4 mm).

In formulating the test specimens, 14 layers of the fiberglass yarn werebraided on the aluminum tools with an application of the resin-fibermixture between each two successive braided layers and also on thesurface of the last braided layer. Thus the composite panels comprised 7plies of fiber (each consisting of two braided layers) and 7 layers ofresin-short fiber mixture. After the fiber layers and resin layers werebuilt up, the resulting panels were cut from the tool, placed on analuminum plate, and then bagged and cured using a technique similar tothat disclosed in Kirk-Othmer, Supplement Volume, at pages 268-270.After bagging, a vacuum of 29" of mercury was applied to the bag and thetemperature was increased to 180° F. and held at these conditions forabout one and one half hours. The curing of the resin was completed overan additional two hour period by increasing the temperature at a rate of2° F. per minute to 350° F. while maintaining an applied pressure of 45psig. After the composite samples were cooled, they were cut into testspecimens which were then analyzed for void content in accordance withADTM D 2734, fiber content by matrix digestion in accordance with ASTM D3171, and short-beam shear failing stress in accordance with ASTM D2344. In addition, selected specimens were tested to determine flatwisetensile failing stress to provide an indication of interlaminarstrength. These tests were carried out using 2"×2" specimens which werebonded on each side to aluminum blocks and then pulled apart in order toarrive at the tensile failing stress.

The results of the test procedures are set forth in Table I. In Table I,the first column gives the amount of short fiber in the resinous matrixmaterial in wt. %, and the second column gives the average length of theshort fibers in the resinous material. The third column sets forth thedensity of the final product in gms/cm³ and the fourth, fifth and sixthcolumns give the resin, fiber, and void contents in volume percents. Thelast two columns set forth the short-beam failing stress and theflatwise tensile failing stress in 1000 psi (KSI).

The short beam failure tests were run in an effort to gain an indicationof interlaminar shear failure between successive resin plies. However,while the failure strengths given in column 7 are believed to give agross indication of sample strength, they are not thought to provide areliable indication of interlaminar strength. The tensile failingstress, on the other hand, is by its nature a direct indication ofinterlaminar strength between plies of fibrous reinforcing material.

The data presented in Table I are average values obtained by testing 2to 5 specimens for each sample run. Standard deviations based upon thesquare root of the average of squared deviations from the arithmeticmean for the average values are given in parenthesis.

                                      TABLE 1                                     __________________________________________________________________________                                           SHORT    TENSILE                           FIBER                                                                              FIBER DENSITY                                                                             RESIN FIBER VOID  BEAM STRESS                                                                            STRESS                        RUN WT   LENGTH                                                                              (gms/cm.sup.3)                                                                      VOL (%)                                                                             VOL (%)                                                                             VOL (%)                                                                             (KSI)    (KSI)                         __________________________________________________________________________     1. 0          1.79  46.1  49.7  4.2   6.08     2.80                                         (.03) (1.57)                                                                              (1.82)                                                                              (1.01)                                                                              (.3)                                    2. 10%  1/4"  1.737 50.5  45.2  4.3   4.69                                                  (.014)                                                                              (1.2) (.03) (1.2) (.14)                                   3. 20%  1/4"  1.78  48.1  48.2  3.66  6.12     2.32                                         (.008)                                                                              (.90) (.11) (.80) (.26)    (.04)                          4. 30%  1/4"  1.74  49.4  45.8  4.8   5.45                                                  (.02) (.60) (.82) (.93) (.22)                                   5. 30%  1/4"  1.82  46.2  50.7  3.1   1.95     2.29                                         (.018)                                                                              (2.45)                                                                              (1.81)                                                                              (.84) (.18)    (.137)                         6. 10%  1/32  1.742 43.1  49.0  7.9   5.59                                                  (.018)                                                                              (.83) (.50) (1.1) (.25)                                   7. 20%  1/32  1.68  39.4  48.3  12.3  2.89                                                  (.02) (.73) (1.08)                                                                              (.59) (.07)                                   8. 30%  1/32  1.70  39.6  48.8  11.6  3.07                                                  (.045)                                                                              (1.12)                                                                              (2.2) (1.42)                                                                              (.27)                                   9. 30%  1/32  1.746 39.0  51.1  9.9   3.95     2.11                                         (.014)                                                                              (.111)                                                                              (.61) (.50) (.11)    (.18)                         10. 20%  1/32  1.81  36.6  54.7  8.6   3.94                                                  (.018)                                                                              (.69) (1.04)                                                                              (.47) (.22)                                  11. 10%  1/64  1.79  45.7  49.5  4.8   6.39                                                  (.013)                                                                              (1.35)                                                                              (.45) (1.14)                                                                              (.25)                                  12. 30%  1/64  1.74  47.2  47.0  5.8   6.43     3.08                                         (.011)                                                                              (.59) (.73) (.15) (.20)    (.14)                             40%  1/64                          5.54     3.50                              50%  1/64                          5.51     2.99                          __________________________________________________________________________

From an examination of the data presented in Table I, and using Run No.1 (zero short fiber content) as a control, it can be seen that thecomposite samples formulated from a resin-short fiber mixture of 1/4"fibers showed a decrease in interlaminar strength as indicated by thetensile failing stress procedure and, although the results are erratic,also a decrease in sample strength as indicated by the short-beam sheartest. The composite samples over the range of fibers tested (from 10 to30 wt.% of the resin mixture) seemed generally to deteriorate withincreasing fiber content. The specimens prepared using the 1/32" fibersshowed test results conforming closely to those for the 1/4" fibers.

For the test specimens prepared using short fibers of a nominal size,1/64", the short beam failing stress showed a modest increase at fiberconcentrations up to 30 wt.% and thereafter a modest decrease withincreasing fiber concentration up to 50%. The interlaminar strength asindicated by the tensile failing stress showed an increase at fiberconcentrations of 30 and 50% and a very substantial increase when theshort fiber content of the matrix resin was 40% fiber and 60% resin.

From the above experimental work it can be seen that the interlaminarstrengths of the composite specimens were influenced substantially bythe length of the short fibers in the resin material and also by theshort fiber concentration in the resin material. The optimuminterlaminar strength occurred at a short fiber length of 1/64", at afiber concentration at a 40/60 fiber-resin mix, corresponding to aweight ratio of short fibers to resin of 0.67.

While the invention is not to be limited by theory, it is believed thatthe interlaminar strength enhancement reflected by the experimental datacan be explained in terms of competing factors involving both theconcentration and the length of the short fibers. The short fiberorientation in the liquid resinous matrix material can be characterizedas being isotropic in nature, in the sense that fiber orientation occursin three dimensions rather than being limited essentially to a planardispersion. Stated otherwise, in describing the fiber orientation byreference to the orthogonal axes of a Cartesian coordinate system inwhich the x and y axes define the laminar dimensions of the compositeand the z axis the cross laminar dimension, the short fibers exhibit asubstantial orientation along the zaxis rather than being confined toorientation in the plane of the x and y axes. Those short fibers havinga substantial z axis coordinate tend to enter into the interstitialspaces between the longer fibers of the fibrous reinforcing plies. Theresin also enters into the interstitial spaces, and the short fibers,together with the resin after it is cured or solidified, tend tostrengthen the matrix between the plies of fibrous reinforcing materialin a manner to enhance the interlaminar strength of the compositestructure. In the optimized use of the short fibers, a sufficient amountis used to strengthen the matrix while not so much as to result in asignificant loss of strength as described below.

The fibers incorporated in the resinous matrix material, because oftheir short length, tend to also act in the nature of fillers so thatincreasing the short fiber content will ultimately result in a decreasein matrix strength, similarly as would be the case with conventionalfillers. However if the short fiber concentration is too low, there arenot sufficient short fibers to function in an effective manner tointerlock the fiber plies together.

If the fibers incorporated into the resinous matrix material are toolong, the fiber orientation will tend to shift from the threedimensional dispersion described above to a dispersion in which twodimensional fiber orientation is favored. That is, fiber orientationalong the x & y axes of the previously referenced Cartesian coordinatesystem is enhanced at the expense of fiber orientation along the z axis.The ultimate of this condition would be the substantially laminardispersion described in the aforementioned patent to DellaVecchia et alin which little or no interlaminar migration of the fibers occurs. Whileas described below, steps can be taken during the formation of thecomposite structure to cause fiber flow across the laminar interfaces,thus enhancing fiber orientation along the z axis and permitting agreater length of the interlocking fibers; for a given set ofconditions, the tendency to orient in the x-y plane will increase withincreasing fiber length.

On the other end of the spectrum, if the fibers are made too short, theyact in essence like fillers, such that they do not function tointerconnect the successive layers of fibrous reinforcing material.Thus, when the short fiber length is decreased from the optimum for agiven set of conditions, overall specimen strength, as well asinterlaminar strength, will tend to be decreased.

Although as noted below, steps can be taken permitting the use of longerfibers, it is preferred in carrying out the invention to employreinforcing fibers in the resinous matrix material having an averageparticle length within the range of 0.2-0.6 mm. The fibers are presentin the matrix material in an amount to provide a weight ratio of shortfibers to resin matrix material within the range of 0.4-1 and morepreferably 0.6-0.8. The short fibers may also be characterized in termsof aspect ratio, i.e. the ratio of fiber length to fiber diameter.Preferably, they will have an aspect ratio within the range of 10-50.While materials such as fillers or other materials to alter thecharacteristics of the composite material may be employed, they shouldbe used with recognition that they may decrease the overall strength ofthe product.

The short reinforcing fibers may be dispersed in the liquid resinmaterial by any suitable procedure. A preferred technique is to dispersefibers of a length greater than the desired final length in the resinmaterial, and then mill the resin-fiber mixture under sufficient stressto reduce the average particle size to the desired range of 0.2-0.6 mm.The milling operation tends to promote isotropic (three dimensional)fiber orientation in the resinous matrix material and in additionprovides for good dispersion of the fibers, that is, it retardsagglomeration of the fibers.

As noted previously, any suitable arrangement of resins (includingthermoplastic and thermoset resins) and fibers may be employed incarrying out the present invention. While it usually will be preferredto employ thermoset resins of the type described previously,thermoplastic polymers may also be employed. Suitable thermoplasticresins which are used in formulating composite structures includepolyetherether ketone, polypropylene, polystyrene, polyether imides andpolyarylene sulfides. In forming composites from such thermoplasticresins, the processing is of necessity carried out at temperatures abovethe melt point of the resin involved. Usually the materials will becomposited under a substantial applied pressure as well; for example, apressure within the range of 15-250 psi.

The fibrous reinforcing materials used in the invention may likewise beof any suitable type employed in forming composite structures. Thereinforcing fibers may take the form of chopped fibers or continuousfiber strands such as employed in unidirectional tapes or braided layupsof the type used in the experimental work described above. In braiding,typically 72-216 continuous fibers are integrated with the shortfiber-resin strengthened matrix. Where chopped fibers are employed, theymay be aligned in a predominantly unidirectional manner or they may berandomly disposed or transversely oriented. Prepreg fiber tapes orfabrics can also be used in carrying out the invention. Where prepregsare used, the short reinforcing fibers as described above may beincorporated into the prepregging resin in addition, or even as analternative, to short reinforcing fibers incorporated into a separatelayer of resinous matrix material.

In accordance with a further embodiment of the invention, fiberorientation across the laminate interfaces, i.e., along the z axis asdescribed above, is promoted, thus permitting use of longer fiberlengths than would otherwise be the case. In this embodiment of theinvention, alternate layers of fibrous reinforcing plies and the matrixmaterial (resin and short fibers) are disposed upon a forming surfacehaving a configuration conforming to that of the desired compositeproduct. The forming surface may be provided by a flat or curved formingtable or by a mold, e.g. an internal mandrel. A pressure gradient isestablished across the built up layers of matrix and reinforcingmaterials to cause the matrix resin and short reinforcing fibers to flowacross the boundaries into the fibrous reinforcing material. Thiscross-laminar flow tends to cause the short reinforcing fibers to orientin the direction of flow thus promoting cross-laminar migration of thefibers.

This embodiment of the invention may be described with reference toFIGS. 1 and 2 which are schematic sectional illustrations taken througha plurality of layers of reinforcing fibers and matrix material disposedupon a suitable forming surface. More particularly and as shown in FIG.1, a forming structure 10 is provided with a plurality of perforations12. The layers of fibrous reinforcing material 14 and shortfiber-impregnated resin 16, as shown, are disposed on the formingsurface of structure 10. As illustrated in FIG. 1, the elongated fibers18 in the fiber ply layers 14 are oriented generally along the length ofthe composite buildup. The short fibers 19 in the matrix layers 16 areshown to be randomly oriented in the resin material.

FIG. 2 illustrates the alternate layers of the composite buildup after apressure gradient is established across the layers from the top tobottom. By way of example, the pressure gradient may be established bypulling a vacuum on the underside of the layup surface 10 to produce alow pressure P₁ while atmospheric pressure (or if desired a higherpressure) P₂ is established on the upper surface of top layer 16. Asindicated schematically in FIG. 2, the liquid resinous material willfollow the established pressure gradient and flow downwardly as viewedin the drawing. This will tend to cause the short fibers 19 in layers 16to become oriented in the direction of flow to facilitate entry of thefibers into the interstitial spaces of the long fibers 19 to lock theseveral plies of reinforcing layers together. After the desiredintegration of the resin and reinforcing fiber layers, the matrix resinis cured to arrive at the final product.

The mechanism illustrated schematically in FIGS. 1 and 2 is to becontrasted with a conventional vacuum bag layup as shown, for example,in pages 268-270 of the Supplement Volume of the Kirk-Othmer. In theconventional procedure, the several plies are simply compressed togetherso that fluid pressure gradients are actually established more or lesshorizontally in the direction of the layers, thus tending to cause fiberorientation horizontally rather than vertically as viewed in FIGS. 1 and2. In the present invention, by establishing a fluid pressure gradientin a direction normal to the fiber and resin layers, as well as bycompressing the layers together, fluid flow and fiber orientation occurin a generally vertical direction.

The various thermoset resins such as those described above undergo asubstantial decrease in volume when they are cured. Because of thisshrinkage factor, warpage of composite structures is commonlyexperienced. While this phenomenon is observed in flat panels, it isparticularly troublesome where composites are employed in forming curvedstructures such as ogival shapes encountered in nose cones or radardomes. For example, in the formation of elongated ogives of the typeused as nose cones for missiles or high performance aircraft, the nosecone structure can be formed in two halves which are then joinedtogether mechanically or by an adhesive to arrive at the final product.While the halves are formed on mold mandrels to very close tolerances,warpage of one or both components of the ogive structure causesdifficulties when the components are joined. Where such curvedstructures such as those used for nose cones and heat shields and thelike are formed integrally in one piece, it is still important thatwarpage be minimized in order to provide a good mating surface forsecuring the shaped curved structure to the remainder of the airframe.

The present invention substantially reduces the warpage of suchcomposite structures. While the invention is not to be limited bytheory, it is believed that the incorporation of the short fibers in thematrix material tends to minimize warpage by displacing a portion of theresin and also because of their interlocking action when extending intothe interstitial spaces between the longer reinforcing fibers. Theinvention is thus particularly applicable to the formation of curvedstructures such as described above including structures in which aplurality of mating parts are formed separately and then joined togetheralong their conforming mating surfaces.

In carrying out this aspect of the invention, each of the matingcomponents is formulated from a plurality of layers of fiber reinforcingmaterial which may be any of the fiber materials described previously.The layers of the fiber reinforcing material are conformed to a desiredshape and integrated with the resinous matrix material comprising amixture of the uncured thermoset resin and the short reinforcing fibers.The desired shape of the component may be arrived at by forming thereinforcing fiber and the matrix material on a suitable mold mandrel.For example, where the final product is a nose cone, each matingcomponent may be 1/2 segment of the cone divided along the axis of thecone. Each component is formed on the mandrel using any of theconventional layup procedures as described previously, or employing thetechnique of FIGS. 1 and 2. After the components are cured and removedfrom their respective mold mandrels, they are then joined along theirconforming mating surfaces to produce the final cone-shaped product. Thecomponent parts may be joined together along their seams by any suitabletechniques such as by mechanical tongue and groove connections and/or byadhesive bonding with an epoxy or other suitable adhesive.

Whereas the present invention has been described with respect tospecific embodiments thereof, it will be understood that various changesand modifications will be suggested to one skilled in the art and it isintended to encompass such changes and modifications as fall within thescope of the appended claims.

I claim:
 1. In a method for forming a fiber reinforced compositeproduct, the steps comprising:(a) establishing a matrix layer of matrixmaterial for said fiber reinforced composite product, said matrixmaterial comprising liquid resin and short reinforcing fibers having anaverage particle length within the range of 0.2-0.6 mm, said liquidresin and said short reinforcing fibers being present in said matrixlayer in an amount to provide a weight ratio of short reinforcing fibersto liquid resin within the range of 0.4-1.0; (b) establishing two fiberlayers of fiber reinforcing material with said matrix layer between saidtwo fiber layers, said fiber reinforcing material having fibers of alength substantially greater than the length of the fibers in the matrixlayer; each of said fiber layers having interstitial spaces; (c)integrating said fiber layers and said matrix layer to cause the liquidresin and short reinforcing fibers of said matrix layer to enter theinterstitial spaces of said fiber layers with said short reinforcingfibers of said matrix layer aligned generally perpendicular to said twofiber layers; and (d) causing the liquid resin from said matrix layer tosolidify to produce said fiber reinforced composite product.
 2. Themethod of claim 1, wherein the weight ratio of said short reinforcingfibers to said liquid resin is within the range of 0.6-0.8.
 3. Themethod of claim 1, further comprising repeating steps (a) and (b) toprovide a plurality of said fiber layers alternating with a plurality ofsaid matrix layers, and integrating said plurality of matrix layers withsaid plurality of fiber layers in accordance with step (c).
 4. Themethod of claim 1, wherein the aspect ratio of said short reinforcingfibers is within the range of 10-50.
 5. The method of claim 1, whereinsaid liquid resin is a thermoplastic resin and is integrated with saidfiber layers at a temperature above the thermoplastic melt point of saidliquid resin and thereafter allowed to cool to produce the solid matrixof said fiber reinforced composite product.
 6. The method of claim 1,wherein said liquid resin is crosslinked after said integration step inorder to produce the solid matrix of said fiber reinforced compositeproduct.
 7. The method of claim 1, wherein said fiber layers areestablished by disposing a layer of said matrix material on a formingsurface of a desired configuration for said fiber reinforced compositeproduct.
 8. The method of claim 1, further comprising the step ofproducing the matrix material of step (a) by dispersing fibers of alength greater than 0.6 mm into said liquid resin and thereafter millingthe resulting fiber resin mixture under sufficient stress to reduce theaverage particle size of the fibers in said fiber resin mixture to avalue within the range of 0.2-0.6 mm.
 9. In a method of producing afiber reinforced composite structure having a desired curved shape, thesteps comprising:(a) providing a plurality of fiber layers of a fiberreinforcing material for said fiber reinforced composite structure, saidfiber reinforcing material including reinforcing fibers havinginterstitial spaces; (b) conforming said fiber layers of fiberreinforcing material to the desired shape of said fiber reinforcedcomposite structure and integrating said fiber layers of fiberreinforcing material with a resinous matrix material for said fiberreinforced composite structure, said resinous matrix material comprisinga mixture of an uncured thermoset resin and short reinforcing fibers ofan average particle length within the range of 0.2-0.6 mm which enterinto the interstitial spaces between said reinforcing fibers of saidfiber reinforcing material with said short reinforcing fibers of saidresinous matrix material aligned generally perpendicular to saidplurality of fiber layers of fiber reinforcing material, saidreinforcing fibers of said fiber reinforcing material having an averagefiber length substantially greater than the average fiber length of theshort reinforcing fibers in said resinous matrix material; and (c)subsequent to the integration of said fiber reinforcing material andsaid resinous matrix material, curing said thermoset resin to cause saidresinous matrix material to solidify to produce said fiber reinforcedcomposite structure of the desired shape.
 10. The method of claim 9,wherein the weight ratio of the short reinforcing fibers to the uncuredthermoset resin within said resinous matrix material is within the rangeof 0.4-1.
 11. The method of claim 10, wherein the weight ratio of saidshort reinforcing fibers to said uncured thermoset resin is within therange of 0.6-0.8.
 12. The method of claim 9, wherein the aspect ratio ofsaid short reinforcing fibers is within the range of 10-50.
 13. In amethod of producing a fiber reinforced composite product having adesired shape from a plurality of mating components, the stepscomprising:(a) for one of said plurality of mating components providinga plurality of layers of fiber reinforcing material for said fiberreinforced composite product, said fiber reinforcing material includingreinforcing fibers having interstitial spaces between the reinforcingfibers; (b) conforming said plurality of layers of fiber reinforcingmaterial to a desired shape for said one of said mating components andintegrating said plurality of layers of fiber reinforcing material witha resinous matrix material for said fiber reinforced composite product,said resinous matrix material comprising a mixture of an uncuredthermoset resin and short reinforcing fibers which enter into theinterstitial spaces between said reinforcing fibers of said fiberreinforcing material with said short reinforcing fibers of said resinousmatrix material aligned generally perpendicular to said plurality oflayers of fiber reinforcing material, said reinforcing fibers of saidfiber reinforcing material having an average fiber length substantiallygreater than the average fiber length of the short reinforcing fibers insaid resinous matrix material; (c) subsequent to the integration of saidfiber reinforcing material and said resinous matrix material, curingsaid thermoset resin to cause said resinous matrix material to solidifyto produce said mating component; (d) duplicating steps (a), (b) and (c)for another of said mating components; and (e) joining the thus producedmating components together along their conforming mating surfaces toproduce said fiber reinforced composite product having the desiredshape.
 14. The method of claim 13, wherein the weight ratio of the shortreinforcing fibers to the uncured thermoset resin within said resinousmatrix material is within the range of 0.4-1.
 15. The method of claim14, wherein the weight ratio of said short reinforcing fibers to saiduncured thermoset resin is within the range of 0.6-0.8.
 16. The methodof claim 13, wherein the aspect ratio of said short reinforcing fibersis within the range of 10-50.